It is well known in the art that ring lasers may be adapted to be used as angular rate sensors. Such ring lasers are known as ring laser gyroscopes. In such a ring laser gyroscope the difference between frequencies of counter-propagating radiant energy in the form of laser beams is a measure of the rate of angular rotation of the structure in which the propagating waves are traveling. Further background and discussion of the basics of ring laser gyro operation may be found, for example, in Killpatrick, "The Laser Gyroscope", IEEE Spectrum, October, 1967; Coccoli, "An Overview of Laser Gyroscopes", P-634 12th Joint Services Data Exchange for Inertial Systems, Norfolk, Va. October, 1978; and U.S. Pat. Nos. 4,383,763 to Hutchings et al. assigned to the assignee of the present application and 4,422,762 to Hutchings et al. assigned to the assignee of the present application, the disclosures of each of which are hereby incorporated by reference.
It is known in the art, as shown in the U.S. Pat. No. 4,422,762 to Hutchings noted above, to dither the mirrors of the ring laser gyro, or some of them, in order to avoid the phenomenon known as lock-in which occurs when the angular rotation rate of the ring laser gyro is close to zero, and is the result of the operating characteristics of a ring laser gyro, e.g., back-scattering of light from the mirrors defining the propagation cavity. In addition, as discussed in that patent, it is known in the art to translate one or more of the mirrors under the control of a piezoelectric transducer, upon which the mirror is mounted, inwardly and outwardly to adjust the propagation cavity length in order to maintain the cavity at a selected integer multiple of the wave length of the laser beam. That patent also discusses a manner of accomplishing this by sensing the AC envelope of the intensity of the counter-propagating beams summed together, which envelope has a peak at an optimum cavity path length, when the path length is an integer multiple of the wave length of the counter-propagating beams.
The piezoelectric transducers used to translate the mirrors to modify the path length have some finite sweep range determined by such things as the range of voltages applied to the piezoelectric transducer, the material of the piezoelectric transducer and the physical structure of the piezoelectric transducer and mirror assembly. Typically, the piezoelectric transducer is capable of translating the mirrors sufficiently to pass through a plurality of, for example, from four to six intensity maxima, i.e., to change the cavity path length through a range of about six integer multiples of the beam wave length.
It is also known that temperature variation due to, for example, changing the environment in which the laser gyroscope is operating, will cause thermal expansion or contraction of the laser gyroscope body and thus the cavity containing the path for the counter-propagating beams. This changes the cavity path length.
The present invention provides for initializing the particular intensity peak selected to operate about, based upon the initial temperature of the gyroscope in order to maintain a selected integer number of wavelengths as the initial cavity path length from start up to start up. In addition, the present invention provides an optimum available movement of the mirrors due to changing the voltage over the available voltage swing for the anticipated range and direction of temperature changes from the start up temperature, over which the gyroscope is expected to operate. This will be referred to herein as selecting the mode for the gyro. It is based upon which of the possible intensity maxima corresponds to a selected path length, and optimizes the expected movement of the path length control mirror or mirrors over the range of temperature changes which the gyroscope is expected to experience in operation, based upon the initial temperature. In this manner, the same path length, (an integer multiple of the laser wavelength), is selected for each start-up, regardless of start up temperature. Ring laser gyro scale factor variables vary with the cavity path length. Scale factors relate the output beat frequency of the counter-rotating beams to the angular variation of the gyro, i.e., gyro input. Therefore, the present invention initiates each operation at a mode which retains the same scale factor variables from start-up to start-up. In this manner, also, the necessity for a mode hop to another peak intensity during the operation of the gyro may be obviated. Such a mode hop would be necessitated, if the changes in the path length resulting from temperature changes are extensive enough to be beyond the capability of the piezoelectric transducers, on the mirror or mirrors used for path length control to translate the mirrors sufficiently to maintain the desired path length.
Recognizing the need for improved cavity path length control, it is a general object of the present invention to provide an improved method and apparatus for maintaining the correct cavity path length in ring laser gyro, including a method and apparatus for initially selecting an optimum operating range for expected temperature variations.
A feature of the present invention involves, initially, slewing the cavity length control mirror or mirrors through their entire operational range and sampling the beam intensity and storing the position of the intensity peaks. Thereafter, a particular peak is selected about which to maintain zero detuning, based upon the initial operating temperature of the gyoroscope.
A further feature of the present invention is to employ a plurality of, for example, three selected operating temperature ranges, for example, one below a temperature T.sub.1, one between a temperature T.sub.1 and a temperature T.sub.2 and one above the temperature T.sub.2. The initial temperature of the gyroscope in these empirically selected temperature bands at the time the system is turned on is employed as a means for optimally selecting a particular peak about which to carry out detuning. The selection results in the initial cavity path length being a selected integer multiple of the laser wavelength and detune being carried out to maintain that integer multiple in this operating mode. Also, the selection results in selecting an initial value of the voltage output to the piezoelectric mirror drive transducers which will allow for a greater change in the direction of high or low temperature response, if the initial temperature is, respectively, low or high, and about an equal range of change in either direction where the start-up temperature is initially in a mid-range. The piezoelectric transducer, therefore, will have sufficient operating range to maintain detune about the selected peak from, e.g., -54.degree. C. to +80.degree. C.
Another feature of the present invention is that for some laser gyroscopes, depending upon various parameters of the structure and operation of the ring laser, may be initially set at the proper mode based solely upon determing the start-up temperature. Where the number of mode peaks occurring over the entire sweep of the CLC mirrors is, e.g., four or less, and/or where the laser exhibits no significant secondary peaks, it may be possible to empirically determine a voltage to which to slew the CLC mirrors which will insure that by engaging the tracking function from such an initial position of the mirrors, the result is to attain the desired mode about which detune is controlled.
Of course, it will be understood that a ring laser gyroscope, according to the present invention, has the capability to slew the path length control mirrors to a position to employ another of the intensity maxima as a detuning point. This might occur, if the temperature change causes a path length variation beyond the capability of the path length control mirrors to correct in the initial mode selected. However, mode shifting results in the injection of inaccurate gyro output data while such slewing occurs. It will also result in having some scale factor error. The present invention is intended to reduce to a minimum the chances of the need for mode shifting occurring during actual operation of the gyroscope in some changing environment.